This invention relates to acoustic wave filters.
One type of acoustic wave filter is the SAW (surface acoustic wave) filter, which makes use of surface acoustic waves. One layout for acoustic wave filters is the impedance element filter. This comprises a number of impedance elements (e.g. one port SAW devices) connected together in a network--for instance in a ladder or balanced bridge network scheme. The impedance elements are connected together electrically but normally do not interact acoustically to any significant extent.
A common network scheme is the ladder network, which is illustrated as a circuit diagram in FIG. 1. FIG. 1 shows a ladder network of impedance elements 1a, 1b arranged between an input point 2, an output point 3 and a ground 4. The network includes impedance elements 1a arranged in series between the input and the output, and impedance elements 1b arranged in parallel between ground and the nodes between the elements 1a. An input voltage is applied between the "hot" input point 2 and ground. The output voltage is taken from between "hot" output point 3 and ground. The number of elements in the network and their impedances may be chosen to give the network the desired electrical properties. The electrical behavior of a ladder network of impedances is well known.
An entire SAW device is commonly provided on a single piezoelectric substrate, with the aim of reducing the size of the device. Small size is important if the device is to be used in compact apparatus such as a hand-held radio telephone. All the impedance elements of a ladder-type SAW filter may be defined on a single substrate by providing a pattern of metallisation (for example vapour deposited aluminum) on a substrate of a suitable piezoelectric material (for example quartz, lithium niobate or lithium tantalate). The pattern of metallisation defines the regions of the substrate that are to serve as the impedance elements. FIG. 2 illustrates such a filter schematically. FIG. 2 shows a three-element ladder-type SAW impedance element filter. Impedance element 5 is arranged between "hot" input 6 and "hot" output 7. Impedance element 8 is arranged between the input end of element 5 and a ground 9. Impedance element 10 is arranged between the output end of element 5 and ground 9. A single, unitary region of metallisation forms the ground. Each impedance element includes an interdigital transducer (IDT) which has a pair of opposed metallised busbars 5a,5b,8a,8b,10a,10b and a set of interdigitated, metallised fingers 5c, 8c, 10c extending from the busbars. When a signal is applied between the "hot" input and ground, surface acoustic waves propagate inside each transducer, through the piezoelectric substrate between the busbars. Reflectors, i.e. reflective configurations of metallisation, (not shown) may be provided to help confine the waves to the transducers. By altering the geometry of the elements (for example by varying the spacing of the busbars and the number, size, spacing and overlap of the fingers) the impedance properties of the transducers and therefore the response of the filter as a whole can be controlled. The filter is connected to other devices off the substrate 12a (for instance elsewhere on circuit board 12b) by wires 13 connected to the input, output and ground.
Ladder-type SAW filters typically exhibit low loss in the passband (less than 2 dB in many cases), reasonable suppression in the stopband on either side of the passband (more than 40 dB in some cases) and acceptable power handling capability.